PURPOSE: To eliminate the need of correctly positioning a phase separation- holding means as a part of assembly work by making the means for holding the separation of a heavy phase from a light phase in a container into a fixed construction.

CONSTITUTION: The two phases having different specific weights in a liq. are separated by centrifugation so as to separate the heavy phase from the light phase. Then, a compartment 12 for the liq. is formed for division by oppositely facing walls 14, 16, 22, 24. Further, as a means inside the compartment 12 for holding the phases in separation regions A, B in the compartment 12 after centrifugation, a porous mechanical means 60 is provided to resist the flow-out of the heavy phase from the portion of the compartment 12 in the presence of shearing forces generated when the light phase is extracted. This means 60 is disposed in the region A of the compartment 12 occupied by the heavy phase. Namely, the need of correctly positioning the phase separation-holding means s a part of the assembly work is eliminated because of its container-fixed and assembled construction.